The present invention relates to a coating material and a honeycomb structure. More specifically, the present invention relates to a coating material capable of improving thermal resistance and chemical durability as well as inhibiting generation of a defect such as a crack by being applied on, for example, a honeycomb structure used for a trapping filter or the like for particulate matter in exhaust gas from an internal combustion engine, a boiler, or the like and dried at high temperature, and a honeycomb structure having an outer wall formed by such a coating material.
There is used a honeycomb structure for a trapping filter or the like for particulate matter, in particular, diesel particulate matter in exhaust gas from an internal combustion engine, a boiler, or the like.
As shown in FIGS. 1A and 1B, a honeycomb structure used for such a purpose generally has a plurality of cells 23 functioning as fluid passages partitioned by the partition walls 24 and has a structure where each of adjacent cells 23 is plugged in one end portion on mutually opposite side in such a manner that the end faces each shows a checkerwise pattern. In the honeycomb structure 21 having such a structure, the fluid to be treated flows into the cells 23 which are not plugged in the inflow hole side end face 25, that is, the cells 23 which are plugged in the outflow hole side end face 26, passes through the partition walls 24, and are discharged from the adjacent cells 23, that is, the cells 23 which are plugged in the inflow hole side end face 25 and not plugged in the outflow hole side end face 26. At this time, the partition walls 24 function as a filter, and, for example, soot or the like discharged from a diesel engine is trapped by the partition walls 24 and deposits on the partition walls 24. The honeycomb structure 21 used in such a manner has an uneven temperature distribution inside the honeycomb structure 21 due to a rapid temperature change or local heat generation to have a problem of crack generation or the like. In particular, in the case that the structure is used as a filter for trapping particulate matter in exhaust gas from a diesel engine (hereinbelow, referred to as a DPF), it is necessary to combust and remove the depositing carbon particulate matter for regeneration, and, at this time, local temperature rise is caused, thereby easily causing problems of deterioration in regeneration efficiency due to uneven regeneration temperature and crack generation due to large thermal stress.
Though damages inside the honeycomb structure due to thermal stress can be inhibited to some extent by segmentalizing and/or by using a material having high thermal resistance such as a silicon carbide based material, the temperature difference between the outer peripheral face portion and the central portion of the honeycomb structure cannot be solved to have a problem of causing a crack in the outer wall of the honeycomb structure.
The Patent Document 1 discloses a ceramic structure having elasticity in the outer wall by allowing the coating material for strengthen the outer peripheral face of the ceramic structure to contain inorganic fibers having a length of 100 μm to 100 mm at 10 to 70 mass % on a solid content basis. However, though damages of the honeycomb structure due to thermal stress can be inhibited by allowing the coating material to contain inorganic fibers, in the case of using long fibers having a length of 100 μm to 100 mm, an organic solvent or a large amount of water is required in order to obtain applicable paste. Since a coating material thus using a large amount of solvent shrinks to a large extent by rapid drying, thereby sometimes causing a crack, the applied coating material cannot be dried at high temperature. Therefore, in Patent Document 1, a ceramic structure where such a coating material is applied is dried at 120° C. However, since a colloidal oxide contained in the coating material is in a state of performing reversible adsorption and desorption of water at 120° C. to have insufficient drying temperature, it has a problem in the points of chemical durability such as water resistance and acid resistance in the outer wall of the resultant ceramic structure.
In the aforementioned Patent Document 1, though the coating material is filled in a depressed portion of the substrate after machining the outer periphery, the inorganic fibers and inorganic particles in the coating material move even right after the application together with the water absorption by the substrate, which may cause a problem of exposure of the substrate in a streak or a small hole-shaped state (hereinbelow expressed as a “sink”). There arise a problem of leakage of soot from the substrate-exposed portion and a problem of causing substrate fracture or the like in the substrate portion due to mechanical or thermal stress. Though the sink can be inhibited by making the inorganic particles coarse, flowability is remarkably deteriorated, thereby causing a problem of making the coating uniformly with no unapplied portion very difficult. On the other hand, though the coating material can be filled into the sink portion by applying the coating material twice, there arise problems of increasing the operation steps and generating a crack by the increase of the thickness.    Patent Document 1: JP-A-2000-102709